The invention relates to a method and device for producing coherent electromagnetic radiation.
A need exists for devices capable of producing substantial power levels of coherent radiation in the millimeter and sub-millimeter wavelength regions. Many high power microwave sources exist which utilize an electron beam as a source of free energy with a resonant structure to convert the electron beam free (kinetic) energy to oscillatory electromagnetic energy. High power devices producing radiation at long wavelengths are available. Low power devices producing radiation at short wavelengths are similarly available. However, devices are not available which are capable of producing substantial power levels at the short wavelengths in the above-mentioned regions.
In the past, proposals have been made to produce coherent microwave radiation by propagating electrons through a dielectrically loaded tube. In Space Charge Waves, by A. H. W. Beck (Pergamon Press, New York, 1958, p. 43), the possibility of such an apparatus is discussed, but dismissed as impractical because of the need for a relatively high dielectric constant material, which material produces poor coupling.
The present invention relates to a practical microwave device of this type which utilizes an electron beam as a source of free energy and a resonant structure for converting the kinetic energy to oscillatory electromagnetic energy. This device utilizes the well-known Cerenkov principle whereby radiation is produced when charged particles traverse a medium with a velocity exceeding the velocity of light in the medium. By utilizing very high speed electrons, at least greater than 0.5 light speed, and preferably greater than 0.8 light speed, the dielectric constant can be relatively low, e.g., 4 or less, so that much better coupling results than in the type of device discussed by Beck.
According to the present invention, a waveguide, preferably of cylindrical cross-section is loaded with a low-dielectric constant material adjacent to an interior path along which the electrons travel. The beam is focussed and guided to destabilize the transverse magnetic mode of the waveguide and to accordingly convert the electron beam energy into high power, high frequency, coherent microwave radiation.
In a first embodiment of the invention, the dielectric constant material has a cylindrical shape, an annular cross-section and surrounds a linear path through the center of the guide along which the electrons move. In a second embodiment, it is the path which is annular in cross-section, and the dielectric load is formed as an annular tube within the annular path and surrounding a central conductor. An electron source, such as a pulse transformer is mounted to supply a beam of electrons which passes along the interior path. Conventional structure is provided about the waveguide for focussing and guiding the beam, e.g., magnetic field coils. The wavelength is terminated in a conventional reflector when the structure is intended to operate as an oscillator, and with a conventional absorbing material when it is intended to operate as an amplifier.
In a further embodiment, a number of the waveguides are coupled together in close proximity to provide high total current capability in a structure whose characteristic transverse dimension is small enough to provide resonance in the sub-millimeter region of the electromagnetic spectrum.
Alternatively, solid dielectric rods can be bundled so that the electrons propagate along the interstices between the rods. Passages can be formed in an otherwise solid block of dielectric material to provide the small transverse dimensions required for producing radiation in the far infra-red spectrum.
In addition to providing substantial power levels, the embodiments of the present invention are voltage tunable and can be modulated. Since the speed of the beam electrons is near the speed of light, a low loss dielectric with a relative dielectric constant not too much greater than unity can be used as the dielectric material and to provide the needed wave slowing which produces the Cerenkov radiation. The power flowing in the dielectric-filled part of the waveguide will not be too much greater than that flowing in the beam filled part.
Other objects and purposes of the invention will be clear from the following detailed description of the invention.